1. Field of the Invention
The present invention relates to a method of decontaminating waste paper (secondary fiber). More particularly, this invention relates to cleaning, for the purpose of recycling, secondary fibers contaminated with non-impact inks and/or stickies by introducing a densifying agent and an agglomerating agent into a slurry of the waste paper fibers, followed by passing the slurry through a centrifugal cleaner.
2. Description of Related Art (Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98)
Waste paper, also known as secondary fiber, has long served as a source of raw fiber material in papermaking. Waste paper materials invariably contain one or more contaminants including inks, dye colors, fluorescent whitening agents, and "stickies" (sticky or tacky contaminants including adhesives, binders, plastic films, coatings, waxes, and the like). Sorted waste paper has had most of these contaminated papers removed and represents a higher, more expensive grade of waste paper. The growing utilization of secondary fibers in many types of paper products has made it necessary for paper manufacturers to process lower grades of waste paper (i.e., unsorted waste paper). While various methods have been employed to remove the contaminants to permit incorporation of the secondary fibers with virgin pulp in papermaking, such lower grade furnish is more heterogeneous and typically contains significantly more contaminants than a higher quality waste paper. Conventional treatment methods may not be adequate to permit incorporation of a significant percentage of unsorted waste papers.
Current approaches to processing recycled fibers can be classified as repulping (slushing of fibers and partial detachment of ink/contaminants from fibers), coarse and fine screening (separation of fibers from contaminants by size and shape), centrifugal cleaning (separation based on density differences relative to fibers), flotation (separation by preferential adsorption of detached ink/contaminants onto air bubbles), washing (separation of small entrained particles from fibers by relative flow of water passing by fibers), and dispersion (reduction in size of ink/contaminants by mechanical action). There is an optimum particle size range for separation of particles from fibers in each of these processes. Depending upon the specific cleanliness requirements for the deinked pulp, it takes a combination of most or all of these processes to cover the size range of particles that one typically encounters. Both the washing and flotation processes depend on the proper use of surfactants. Depending on the relative strength and size of the hydrophillic versus hydrophobic portion of the surfactant molecule, the surfactant will cluster around ink and other contaminant particles, rendering the particles hydrophylic for washing purposes or more hydrophobic for flotation. The opposing natures of washing surfactants and flotation surfactants can cause problems in combination flotation/washing systems.
Certain specific removal approaches have been disclosed for specific waste paper contaminants:
U.S. Pat. No. 5,211,809 discloses removing color from dyes from secondary pulps with non-chlorine based bleaching agents in treating sequences using oxygen with combinations of peroxide, ozone, and/or hydrosulfite at controlled pH conditions.
U.S. Pat. No. 5,213,661 teaches using oxygen to reduce the tackiness of stickies in secondary pulps and, optionally, using oxygen with alkali and/or detackification agents for optimum stickies control. U.S. Pat. No. 5,080,759 teaches introducing a water-soluble organotitanium compound into the water system of a papermaking process containing the secondary fiber to reduce the tackiness and adhesive properties of the stickies contaminants.
Also, published Japanese Patent Application No. HEI 3[1991]-199477 teaches a method of recycling used paper containing either fluorescent white paper or colored paper or both by introducing ozone into a dispersed slurry of said used paper.
The effectiveness of contaminant (ink/stickies) removal strategies usually is determined by a combination of TAPPI dirt counts (to quantify larger specks), brightness readings, and image analysis. Image analysis involves the imaging of the surface of a handsheet or piece of paper, followed by digitization of this image by a computer. The resolution is set by specifying the minimum number of pixels that a particle must cover to be represented accurately. The output from the analysis is the total area of contaminant particles above a resolvable size on the surface of the paper sample. The limit of visibility for particles is about 40 micrometers. The standard limit for writing printing grades is less than 5 ppm.
While waste paper contaminants such as dyes, whiteners, and stickies present real problems in recycling, the most common removal problem is with inks. Printing inks have been classified broadly as impact and nonimpact inks.
Impact inks are used in conventional printing processes such as letterpress, flexography, and lithography. These inks are pressed or laid onto the paper but do not fuse with it. They generally consist of a pigment suspended in an oil-based alkaline aqueous medium. The paper industry has been successfully deinking papers containing impact inks for years using washing and/or flotation type systems.
Also, U.S. Pat. No. 4,381,969 teaches bleaching waste paper containing encapsulated constituents such as inks by repulping the waste paper in the presence of an aqueous alkaline solution which contains a peroxide compound such as hydrogen peroxide.
Other patents disclosing deinking methods include:
U.S. Pat. No. 4,013,505, "Method of Deinking Printed Wastepapers"; PA0 U.S. Pat. No. 4,076,578, "Ink Removal From Waste Paper"; PA0 U.S. Pat. No. 4,147,616, "Apparatus for Deinking Printed Wastepaper"; PA0 U.S. Pat. No. 4,780,179, "Method for Producing Pulp from Printed Unselected Waste Paper"; PA0 U.S. Pat. No. 5,151,155, "Process for Deinking Wastepaper with Organically Modified Smectite Clay"; PA0 U.S. Pat. No. 5,221,433, "Deinking Wastepaper Using Alkoxylation Product of Carboxylic Acid Containing an OH Group and Alkylene Oxide"; PA0 U.S. Pat. No. 5,225,046, "Wastepaper Deinking Process"; PA0 U.S. Pat. No. 5,227,019, "Wastepaper Deiitling Process"; PA0 U.S. Pat. No. 5,228,953, "Deinking Waste Paper Using a Polyglycol and a Phosphoric Ester Mixture"; and PA0 U.S. Pat. No. 5,238,538, "Method for Deinking Recycled Fiber by Applying Direct Current Electric Field." PA0 U.S. Pat. No. 4,276,118, "Deinking Waste Electrophotography Copy Paper"; PA0 U.S. Pat. No. 4,561,933, "Xerographics Deirking"; PA0 U.S. Pat. No. 5,141,598, "Process and Composition for Deinking Dry Toner Electrostatic Printed Wastepaper"; and PA0 U.S. Pat. No. 5,217,573, "Removal of Laser Printer and Xerographic Ink from Recycle Paper."
Increasing amounts of secondary fiber, however, are generated from papers subjected to reprographic printing processes such as electro-photographic copying (e.g., xerography) and laser printing. These printing methods employ nonimpact inks. Nonimpact inks are comprised of a pigment and a thermoplastic resin. The resin is a bonding agent which fuses the pigment to the sheet and to other pigment particles. The pigments employed in nonimpact inks can be categorized as either iron-based or non-iron based (e.g., carbon based). The resin polymers become cross-linked and resistant to chemical and mechanical action, making nonimpact printed papers difficult to deink by conventional deinking processes. Once detached from the fibers the toner ink particles tend toward a size which is larger than that which can be efficiently handled by flotation or washing and too small to be removed by cleaners and screens. Various approaches have been disclosed specifically to remove reprographic type inks:
Conventional deinking processes require high energy input and employ additives or solvents to assist the removal of reprographic (nonimpact) type inks. Significant fiber loss normally results along with the ink removal. To employ this class of papers economically for recycling to higher grade, bright papers, a method is needed which removes the inks while retaining the fiber. Unfortunately, known ink removal processes, when employed to remove nonimpact inks, have the following common disadvantages:
high fiber loss (20-25%) PA1 high solid waste PA1 high capital costs (due to large equipment requirement) PA1 low ink removal efficiency
The polymerized nature of fused xerographic and laser inks from recycle non-impact printed papers which make such papers difficult to deink by conventional washing and flotation methods is that a significant portion of the detached laser ink particles, upon repulping, will be larger than 150 microns; and, therefore, will not be effectively removed by washing and flotation systems. The shape of the individual particles is generally flat and plate-like with densities slightly less than water, so centrifugal cleaning is also not very effective. Their shapes, in addition to their relatively small size, cause removal difficulties in screens and cleaners (Only the larger particles will be effectively screened.). While dispersion (via refining) can reduce the size of the ink particles to provide more effective removal by washing, such refining is energy intensive, and it increases the drainage problems already inherent in the use of recovered fiber on paper machines.
A chemical approach to this problem has been to chemically modify the laser inks to allow their removal in standard forward cleaners. The process changes the shapes of the particles from plate-like to spheres, and the density of the spheres is increased to values in the range of 1.0 to slightly above 1.0 g/cc. The change of shape and enhanced density allow for improved removal of these inks by conventional forward cleaners. The disadvantage of this chemical approach is that ink removal efficiencies are in the range of 95-97%, with high levels of the smaller ink particles eluding chemical densification and removal.
A novel approach to overcome the above-noted disadvantages of conventional ink removal methods for reprographic ink removal is provided in U.S. Pat. No. 5,527,426, published Jun. 18, 1996 and commonly owned with the instant application. The patentees teach adding magnetite and an agglomerant to the repulped slurry so that non-iron based inks, as well as iron based inks, can be magnetically removed. Industrial magnets, however, are not standard equipment in most paper mills; so a more desirable method of ink contaminant removal would be one which relied on traditional paper mill equipment.
While waste paper contaminants such as xerographic inks present real problems in recycling, stickies present unique problems due to their non-magnetic character and to their different chemistry and physical properties (e.g., high tackiness) compared to both impact and nonimpact inks, whether magnetic or non-magnetic. In paper recycling, sticky contaminants, such as hot melt adhesives, latex, pressure-sensitive adhesives, and wax, pose considerable problems to papermakers. When present in unacceptable amounts, stickies can cause problems with both paper machine operation and product quality. This problem has been addressed recently by the prior art.
In U.S. Pat. No. 5,639,346, published on Jun. 17, 1997 and commonly-owned with the instant application, the patentees disclosed a method of magnetic removal of stickies by introducing magnetite and an agglomerant to facilitate removal of the agglomerated magnetite- stickies particles by passing the slurry by a magnetic field. Again, such a solution requires additional, non-standard paper mill equipment.
The object of this application is to provide a novel decontamination process for application to waste paper furnishes containing either ink contaminants, whether iron-based or noniron-based, or stickies contaminants, or both. It is a further object of the invention of this application to provide a deinking process solely reliant on conventional equipment. Particularly, it is an object of this application to provide a centifugal cleaning process of improved efficiency in the removal of nonimpact printing inks or stickies or both.